Oxiranic ring

The opening of the oxirane ring is accompanied by inversion except when the oxirane ring is in the terminal position of an aliphatic chain the ultimate result is equivalent to trans addition to the double bond. Thus cyciohexene yields trans-1 2- f/ohexanediol ... 

It is also possible to convert carbonyl groups into oxirane rings with cenain carbenoid synthons. The classical Darzens reaction, which involves addition of anions of a-chloroacetic esters, has been replaced by the addition of sulfonium ylides (R. Sowada, 1971 C.R. Johnson, 1979). 

The oxirane ring-opening reaction requires the presence of a basic catalyst. An acidic catalyst also works, but the polymerization of the oxirane limits its usehilness. In the case of 2-mercaptoethanol (eq. 8), the product has been found to be autocatalytic, ie, the product is a catalyst for the reaction. 

The geometries of oxiranes have been determined mainly by X-ray diffraction on crystalline natural products, the oxirane ring being widespread in nature (Section 5.05.5.3). However, the detailed structure of the parent compound (Figure 1) has been secured by microwave spectroscopy and electron diffraction studies (64HC(l9-l)l). The strain in this... 

A relatively few oxirane ring openings have been reported to give retention or a mixture of inversion and retention (see the general reviews on the mechanisms of oxirane ring opening, and the specialized one devoted to retention (68RCR448)). 

DNA binding of (30) and its analogs depends on nucleophilic attack on the oxirane ring at C(10), the preferred position because it is benzylic. With strong nucleophiles such as... 

A few natural products with oxirane rings fused to five-membered rings are known. These include the antibiotic methylenomycin-A (82) (79H(13)353, 79JOC4210,80JA3904, 81CC714) and the truly remarkable fungal metabolite trichoviridine (83), which appears to be the first example of an isocyanide epoxide (76CPB832). 

Acidic cleavage of the oxirane ring in (210) results in formation of the alkynic aldehyde (211) (68TL4905). 

Natural products biosynthesis, 1, 83-109 fused oxirane rings, 7, 192 fused thiirane rings, 7, 192-193 as insecticides, 1, 198 nomenclature, 1, 28-31 oxiranes, 7, 120 as pharmaceuticals, 1, I46-I56 pyridine derivatives... 

Characterisation of the antibiotic monordene (also referred to as radicicol) with the elemental composition Cig/Zz/OgCl isolated from Monosporium bonorden gave the macrolide structure 1. The relative configuration of the H atoms on the two conjugated double bonds (6,7-cis, %,9-trans-) could be deduced from the 60 MHz H NMR spectrum The relative configuration of the C atoms 2-5, which encompass the oxirane ring as a partial structure, has yet to be established. 

In nuclear magnetic resonance (NMR) spectra the protons of the oxirane ring are usually shifted out of the steroid methylene envelope. Tori et al have tabulated the relationships of the angular methyl proton resonances and the oxirane proton signals with respect to location and configuration of the oxirane group. 

The alkoxide formed by attack of the carbonyl group of perfluorobutyrolactone opens the oxirane ring of hexafluoropropene oxide at the central carbon atom [37] (equation 33). A fluorinated sultone reacts with halogenoalkanes in the presence of metal fluoride [3S] (equation 34). 

Reaction of epichlorohydrin with I-naphthol affords the glycidic ether (61). Opening of the oxirane ring with isopropyl-iimine gives racemic propranolol (62) the only 3-blocker... 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

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